1. Field of the Invention
The present invention relates to a digital logarithmic function generator for generating a fragmental graph approximated logarithmic function with time as a variable, by performing frequency modulation under digital control. The function generator of the present invention is useful in various control systems, such as an electronically controlled fuel injection system, which require electrical signals logarithmically related to time.
2. Description of the Prior Art
The prior art logarithmic function generators of the above type include, for example, an analog type device which makes use of a logarithmic voltage to current characteristic of a diode to produce an output voltage which is logarithmically related to an input voltage, and a digital type device which generates an approximation of a logarithmic function by a progression development of the logarithmic function. In the analog type device, because of the non-uniformity in the diode characteristic, it is difficult to obtain a logarithmic function generator of uniform characteristic. In addition, since the diode characteristic is apt to be changed with temperature, there is a drawback in that the logarithmic function voltage generated is unstable.
In the digital type device, there are drawbacks in that complex arithmic operation circuits are required to execute the development formula and a large-scale and complex circuit configuration is required to accomplish a high precision operation.